modules: systemd_failed_units: Introduce RequestFailedUnits member

Split-out request of failed units from systemd into a separate member
function.

This increases the readability and extendability, but preserves the
current functionality (non-functional change).

Signed-off-by: Steffen Kothe <steffen.kothe@skothe.net>

include/modules/systemd_failed_units.hpp

@@ -25,6 +25,7 @@ class SystemdFailedUnits : public ALabel {
 
   void notify_cb(const Glib::ustring &sender_name, const Glib::ustring &signal_name,
                  const Glib::VariantContainerBase &arguments);
+  void RequestFailedUnits();
   void updateData();
 };